Modulation of b-axis thickness within MFI zeolite: Correlation with variation of product diffusion and coke distribution in the methanol-to-hydrocarbons conversion

Abstract

The conversion of methanol-to-hydrocarbons (MTH) has been studied over a series of Zn/ZSM-5 zeolites with different thickness of b-axis, as well as similar lengths of a-axis. It has been demonstrated that the decrease of b-axis thickness from 220 nm to 2 nm leads to the remarkably longer lifetime, accompanied by the shift of selectivity toward trimethylbenzene and increased coke tolerance capacity. Methylbenzenes, as the intermediate product of the aromatic-based cycle, can diffuse out of the straight channels in the Zn/ZSM-5 nanosheet quickly, suppressing the aromatic-based cycle. The evolution of coke species, including the quantity, types, and location, as a function of the reaction time, has been systematically investigated. During the initial reaction period, the coke preferentially forms in mesopores and then is deposited mainly in micropores as the reaction proceeds. The Zn/ZSM-5 nanosheet shows a crystal face dependency on coke deposition, which mainly occurs on the (010) surface because the diffusion of product molecules is found to be only through the straight channels. But, for the conventional Zn/ZSM-5 catalyst, the diffusion of small molecule products is through both channels, while aromatics are only diffused through the straight channels, which leads to the significant difference of coke densities and sp2/sp3 ratios on the (010) and (100) surfaces. The adsorption simulations indicate that butene and p-xylene are the largest molecules which diffuse through the sinusoidal channels and the straight channels, respectively.